1. Field of the Invention
The invention relates to a color image pickup device, and, more particularly, to a color image pickup device having color temperature converting filters.
2. Related Background Art
To correctly photograph the color of an object by a color video camera, it is necessary to correct spectral characteristics of an illuminating light source. FIG. 1 shows spectral radiating characteristics in the case where a color temperature of the light source is changed. An axis of abscissa indicates a wavelength of the light and an axis of ordinate indicates a relative value of a radiation energy. As will be understood from FIG. 1, the spectral characteristics of the light source largely change in dependence on the color temperature.
In the conventional color video camera of the three-color (RGB) separation type, there is used a circuit construction such that level differences among the channels of R, G and B are detected and gains of the respective channels are adjusted so as to eliminate the level differences among the channels in the achromatic color photographing operation. With this circuit, the spectral characteristics of the light source are corrected. Such an adjustment is called a white balance adjustment. A circuit to automatically execute the white balance adjustment is called an automatic white balance adjusting circuit and is well known.
FIG. 2 shows gain correction amounts of the R and B channels for the color temperature of the light source. An axis of abscissa indicates a color temperature of the light source and an axis of ordinate indicates a correction gain by a unit of dB. In the example, a temperature of 3200.degree. K. is used as a reference.
However, in the conventional automatic white balance adjusting circuit, when it is intended to cope with illumination color temperatures of a wide range, a gain correction amount increases and a difference in S/N ratio, dynamic range, or the like occurs between the channels, so that such a method is unpreferable. On the other hand, there is a method of auxiliarily using a color filter (color temperature converting filter) to convert the color temperature of the light source. Three or four kinds of color temperature converting filters are prepared and a proper color filter is attached in accordance with the light source. FIG. 3 shows a change in gain correction amount depending on the color temperature when the color temperature converting filter is used. As will be understood from FIG. 3, the gain correction amount decreases.
As mentioned above, by auxiliarily using the color temperature converting filter, the gain correction amount in the automatic white balance adjusting circuit can be reduced. However, such a method is also limited to the case where the selected color temperature converting filter is proper to the light source. When the selected color temperature converting filter is improper to the light source, the gain correction amount in the automatic white balance adjusting circuit doesn't decrease and an advantage of the color temperature converting filter is not derived.